Secondary explosions on silicic lavas: A case study from Banco Bonito rhyolite lava flow, New Mexico

While secondary explosions are well documented in basaltic and pyroclastic deposits, they have rarely been studied on silicic lava flows. The Banco Bonito rhyolite flow (BBF), the youngest eruptive product of New Mexico's Valles Caldera, is dotted with numerous large explosion craters. We document the characteristics of these features and evaluate mechanisms of formation to advance the conceptual model for secondary explosion on silicic flows. Fifty-six explosion craters range from 62 to 243 m in diameter and 3 to 29 m in depth, with volumes on the order of 10 4 –10 6 m 3 . They are circular to mildly elliptical (median ellipticity = 1.3) with pronounced ejecta rims and are spatially correlated with arcuate, discontinuous pressure ridges that span much of the lava surface. Textural zonation within the lava flow indicates potential locations for steam accumulation. A coarsely vesicular pumice (CVP) horizon capped by an impermeable rigid crust may have enabled the buildup and pressurization of steam, thus driving the explosions. Field-based textural mapping reveals that the CVP layer varies in thickness and depth across the flow, potentially accounting for observed crater variability. Notable disparities in volume between craters and their ejecta rims suggest a significant fraction of material was expelled distally, beyond the rim or into a transient ash cloud. Modeling ash clouds as instantaneous thermal plumes yields a median height of ~5 km, potentially occurring many years after flow emplacement, highlighting the scale of these events and their associated volcanic hazards.